Spontaneous Imbibition in Nanomatrix-Fracture of Low Permeability Using Multiscale Nanofluidic Chips.

Spontaneous imbibition has garnered increasing attention as an attractive mechanism for developing tight reservoirs. Despite valuable insights from previous experiments, there remains a lack of understanding regarding the imbibition process within multiscale nanopore-fracture networks. In this work, we devised an innovative multiscale model incorporating over 105 nanochannels and integrating a microfracture network to explore the complex imbibition behavior in tight formations. Additionally, fracture-free nanomatrix models with low permeability were developed for comparative discussions. The results show that the Lucas-Washburn equation remains valid at the tremendous fracture-free nanopore networks under the confinement of 500 nm, with a relative deviation of ±6%. The nanomatrix's heterogeneity hinders the imbibition rate, resulting in a reduction of 4.6 to 10.8% in the imbibition slope. The viscosity plays a dominant role in the change of imbibition slope as temperature varies. Our experiments also found that the interactions between the nanomatrix and bulk fracture complicate the imbibition process. A single wetting front no longer applies in the nanomatrix-fracture networks. Differing fracture/microchannel connectivity leads to disparities in macroscopic patterns, saturation rates, and flow directions. The spatial arrangement of fractures significantly impacts the imbibition time. Overall, this work based on nanofluidic techniques systematically explores the effects of matrix heterogeneity, temperature, and fractures on the imbibition process. The real-time in situ visualization of fluid distribution in multiscale matrix-fracture systems has been achieved, which offers theoretical guidance for practical engineering applications.